Schools across Canada have responded to the growing number of children vulnerable to anaphylactic shock by declaring themselves “peanut-free zones”.

Parents have been asked to ensure that children who eat peanut butter for breakfast wash their hands before coming to school. Such extreme measures are necessary, school officials and immunologists say, to protect the 1-5 per cent of pupils susceptible to anaphylactic shock.

Unlike most food allergies,which result in rashes or sinusitus, anaphylactic reactions can progress rapidly from itchy lips or eyes to coma and death.

School boards could also find themselves in a legal bind if they failed to take precautions as they are required to provide “a reasonable education”, Eleanor Doctor, a Vancouver lawyer, has warned.

According to Dr Michael Goldman of the Johns Hopkins University, Baltimore, microscopic quantities of peanuts, as little as 0.25mg or about 1800th of a peanut, may be enough to trigger reactions. In 1994 an Ontario student on a field trip died after eating a cheese sandwich that had been packed in a bag with a peanut-butter sandwich.

Vancouver’s Carnarvon community school’s experience is typical. According to vice-principal Isabel Grant: “Last year when banning peanuts was a hypothetical issue, there were heated discussions within the administration and the school council. Since becoming a ‘peanut-free zone’ this year, there hasn’t been the backlash that I would have guessed, even from those who routinely brought peanut-butter sandwiches.”

She credits this to the school’s decision to explain the issue to parents and the help they gave them in planning alternative school lunches. Maintaining “peanut-free zones” is complicated by the fact that the presence of peanuts and peanut products is not always obvious.

The Allergy, Asthma and Immunology Society of Ontario has warned that peanut butter may be used to thicken commercial chilli or to seal egg rolls.